1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas discharged from internal combustion engines such as those used in automobiles. More specifically, it relates to a catalyst for purifying an exhaust gas by purifying a NO.sub.x -containing gas having a relatively high residual oxygen content, such as an exhaust gas discharged from automobiles under a lean combustion operation condition, wherein the air-fuel ratio is relatively higher than that of a theoretical air-fuel ratio.
2. Description of the Related Art
A catalyst capable of conducting an oxidation of, for example, carbon monoxide, hydrocarbons, simultaneously with a reduction of nitrogen oxides (NO.sub.x) is generally used as a catalyst for purifying an exhaust gas discharged from internal combustion engines for automobiles and from industrial plants. Typical examples of such a catalyst known in the art include those prepared by coating a .gamma.-alumina slurry on a refractory support, such as cordierite and calcining the coating and supporting thereon a noble metal, such as palladium, platinum or rhodium, or any mixture thereof. Many proposals have been made for an enhancement of the catalytic activity; for example, Japanese Unexamined Patent Publication (Kokai) No. 61-11147 discloses a catalyst comprising a .gamma.-alumina particle stabilized with a rare earth metal oxide and a noble metal or the like dispersed thereon, wherein rhodium is dispersed on a particle substantially free from a rare earth oxide.
The conversion efficiency of the above-mentioned catalysts, however, is greatly influenced by the set air-fuel ratio of the engine, and a large amount of oxygen is present on the lean side, i.e., in the lean mixture, even after combustion, so that the oxidation is actively conducted but little reduction occurs. In contrast, on the rich side where the air-fuel ratio is low, since the amount of oxygen becomes small after combustion, the oxidation is inactive but the reduction is high. The catalyst functions most effectively around a theoretical air-fuel ratio (A/F=14.6) wherein a good balance between the oxidation and the reduction is provided, which has led to the use of a feedback control wherein the oxygen concentration of the exhaust system is detected to maintain the ratio of the air-fuel mixture at a theoretical air-fuel ratio.
Under the above-mentioned circumstances, a proposal has been made for a catalyst for purifying an exhaust gas which can remove NO.sub.x through a reduction even on the lean side (see Japanese Unexamined Patent Publication (Kokai) No. 1-130735). This catalyst comprises a zeolite ion-exchanged with a transition metal and can remove NO.sub.x with a high efficiency even in an oxygen rich atmosphere on the lean side where the air-fuel ratio is high.
A catalyst for combustion of a carbonaceous substance, comprising a zeolite type copper aluminosilicate containing a copper ion in the zeolite structure thereof is disclosed in, for example, Japanese Examined Patent Publication (Kokoku) No. 57-36015).
The above-mentioned zeolite catalyst ion-exchanged with a transition metal, such as copper, effectively reduces NO.sub.x even on the lean side. According to studies conducted by the present inventors, however, the NO.sub.x conversion was apt to become lower under actual service conditions or in an emission requirement mode, and the reason for this is believed to be as follows. For example, when a copper ion is deposited at a site other than the ion exchange sites of the zeolite, the copper becomes a copper oxide cluster during the steps of drying and calcination in the production of a catalyst. The copper cluster completely oxidizes hydrocarbons (HC) through the following reaction, which causes the amount of hydrocarbons used in the reduction of NO.sub.x to be reduced, whereby the NO.sub.x conversion is lowered. EQU 2HC.fwdarw.H.sub.2 O+CO.sub.2
Since this reaction becomes particularly significant at a high temperature, the NO.sub.x conversion may be further lowered under actual service conditions or in an emission requirement mode. Further, since the zeolite structure of the zeolite catalyst may be broken with an elapse of time, the catalyst has a problem of durability.